The conventional obstacle determination device for vehicles is disclosed in JP-A-2006-117157 corresponding to U.S. 2006/0087417 Al as a pedestrian collision detecting device for detecting collision of a pedestrian or the like with a vehicle. The obstacle determination device includes an absorber, a pressure sensor, a velocity sensor and an airbag ECU device. The absorber is a member for absorbing an impact generated by collision of an object with a vehicle. The absorber includes a chamber member and a connection member. The chamber member is attached to a front surface of a bumper reinforcement by the connection member. Air is filled in a chamber which is provided by the chamber member. The pressure sensor detects a pressure inside the chamber. The pressure sensor is connected to the connection member through a tube. The velocity sensor is a sensor for detecting a velocity of the vehicle and arranged inside the vehicle. The pressure sensor and the velocity sensor are connected to the airbag ECU. The airbag ECU determines based on detecting results of the pressure sensor and the velocity sensor whether collision with a bumper of the vehicle is caused by a pedestrian. Specifically, the airbag ECU determines whether the pressure in the chamber exceeds a threshold value determined by a running velocity of the vehicle.
The chamber member which provides the absorber may be made of resin. The absorber and the chamber member may be made separately, and both may be made of resin. Hardness of resin changes with temperature. The resin becomes soft at high temperature and hard at low temperature. Thus, when the same obstacle collides with the vehicle under the same condition, the deformation degree is different depending on temperature. Hereby, a detection result of the pressure sensor changes. The pressure becomes high at high temperature and low at low temperature. Therefore, when temperature changes significantly, it is difficult to determine accurately whether the collided obstacle is a pedestrian.